The present invention relates generally to the field of photobioreactors. More particularly, it concerns a photobioreactor, a culture unit and a process for cultivating photosynthetic organisms, such as microalgae.
The algae biomass artificially produced is usually dried and used as a nutraceutical food for humans. Derived fine biochemical products can be extracted from algae, for instance, cosmetic pigments, fatty acids, antioxidants, proteins with prophylactic action, growth factors, antibiotics, vitamins and polysaccharides. The algic biomass can also be useful, in a low dose, to replace or decrease the level of antibiotic in animal food or be useful as a source of proteins. Furthermore, the algic biomass provided in a wet form, as opposed to a dried form, can be fermented or liquefied by thermal processes to produce fuel. The algae biomass which may have commercial interests are: Spirulina maximum, Spirulina platensis, Dunaliella salina, Botrycoccus braunii, Chlorella vulgaris, Chlorella pyrenoidosa, Serenastrum capricomutum, Scenedesmus auadricauda, Anabaenopsis, Aulosira, Cylindrospermum, and Tolypothrix.
Various approaches of algae production are known in the art. A first generation of photobioreactors is based on the use of shallow lagoons agitated with one or several paddle wheels. The photobioreactors of this first generation have the disadvantage of offering poor productivity to the seasonal and daily climatic variations and are thus to be confined to tropical and subtropical areas. They also have the disadvantage of being prone to contamination.
Other approaches of algae production have emerged over the past years. An example, is the use of closed cultivating systems which have gained popularity because they overcame the majority of the limitations allotted to the conventional shallow lagoons. The most popular closed cultivating systems are the tubular photobioreactors whose configuration allows to reach high production rates due to the optimization of their light path, their temperature control and their culture mixture. This second generation of photobioreactors allows for an automated control and a more effective absorption of CO2 used as a source of carbon. It also allows the pH of the culture medium to be lowered. Examples of tubular photobioreactors are shown in U.S. Pat. Nos. 5,137,828; 5,242,827 and 6,174,720.
The photobioreactors of the first and second generations were constructed to principally receive the sun""s daylight. Their productivity is indeed limited to the intensity of the sun, which intensity depends on the photoperiod, the season, the localization and the diurnal cycle. It is possible to provide an artificial light to compensate for the periods of low intensity. However, in such a case, the energy losses are numerous. The fact that these types of photobioreactors are being laid out outside, even under a greenhouse, also limits their use in more moderate climatic areas.
The use of artificial light as an energy source for the growth of microalgae was the subject of several studies and gave birth to the third generation of photobioreactors. Photobioreactors of various shapes and employing various systems of artificial lighting are known in the art. Examples of these photobioreactors are given in U.S. Pat. Nos. 5,104,803; 5,169,051 and 5,614,378. Because their scaling was too expensive, the photobioreactors of the third generation rarely exceeded the stage of prototype. Furthermore, a major drawback with these photobioreactors, is that they become dirty or contaminated unless special precautions are taken. Indeed, adhesions of microalgae occur in a natural manner, particularly on the walls where light is emitted. The extent of this phenomenon is a function of the cultured algae species, as well as the constituent material of the light-emitting devices and the culturing conditions. This effect of adhesion of microalgae leads to a reduction in the volume of culture exposed to the light. It also increases the risks of contamination as a result of the development of bacteria and/or protozoa, which develop in the absence of light.
On an other hand, it is now of general knowledge that the main gas causing the greenhouse effect and the reheating of the planet is the carbon dioxide (CO2). This gas comes from various sources. CO2 of anthropic origin is emitted by breathing, fossil combustion of fuel and by certain chemical processes. It is also shown that the inorganic carbon provided in the form of gaseous CO2 or of bicarbonate can be useful as the only source of carbon for the growth of the microalgae. The gaseous CO2 is generally directly injected into the culture medium at concentrations reaching 15%, the balance consisting of air. Though more expensive, the bicarbonate, generally provided in the form of sodium bicarbonate, is another source of inorganic carbon assimilable by the microalgae.
Several drawbacks were identified concerning the coupling of techniques involving the sequestration by the microalgae of CO2 of anthropic origin. The most important drawbacks are the dependence on the light intensity of the sun, the external temperature, the large surfaces occupied by basins of low yield culture and the CO2 absorption towers.
Although many photobioreactors have been proposed in the prior art, there is still a need for an improved photobioreactor using artificial light as the energy source for photosynthesis. Indeed, there is a need for a photobioreactor designed so as to reduce or eliminate the problem described above concerning the accumulation of microalgae on the light-emitting source. There is also a need for a photobioreactor that can easily be coupled with techniques involving the recycling of CO2 of anthropic origin.
An object of the present invention is to provide a photobioreactor that satisfies at least one of the above-mentioned needs.
According to the present invention that object is achieved with a photobioreactor comprising a container for containing a liquid culture medium for cultivating photosynthetic organisms, and a plurality of parallel light-emitting tubes mounted within the container and extending in a first direction, each light-emitting tube having an outer surface. The photobioreactor further comprises cleaning means mounted within the container for cleaning the outer surface of the light-emitting tubes, and actuating means for actuating the cleaning means.
Thanks to the cleaning means provided within the container and the actuating means for actuating the same, it is possible with the present invention to easily get rid of the cultivated organisms which may block the light source by adhering to the same. Also, the simplicity of its concept makes it a very attractive and easy tool to be used for growing a desired organism at a substantially low cost.
According to another aspect of the invention, there is provided a culture unit for cultivating photosynthetic organisms, comprising a photobioreactor for cultivating a photosynthetic organism in a liquid culture medium and a bioreactor for producing bicarbonate ions and hydrogen ions from a CO2-containing gas, the bioreactor comprising:
a reaction chamber containing immobilized carbonic anhydrase or analog thereof capable of catalyzing the hydration of dissolved CO2 into the bicarbonates ions and hydrogen ions,
a liquid inlet in fluid communication with the reaction chamber, for receiving a liquid,
a gas inlet in fluid communication with the reaction chamber, for receiving a CO2-containing gas; and
a liquid outlet in fluid communication with the reaction chamber, for dispensing a liquid solution containing the bicarbonates ions and hydrogen ions.
The culture unit further comprises means for transferring the solution of bicarbonates ions and hydrogen ions dispensed from the liquid outlet to the photobioreactor.
As can be appreciated, one advantage of a culture unit as defined above is that it allows the use, at low cost, of bicarbonate ions as the source of carbon necessary for the growth of the organisms. Also, thanks to the use of a CO2-containing gas in the bioreactor, the unit has the advantage of reducing CO2 contained in the air. Consequently, it also helps reducing the greenhouse effect mentioned above.
The invention also proposes a process for producing photosynthetic organisms, the process comprising the steps of:
a) cultivating a photosynthetic organism in a photobioreactor as defined above, and thereby obtaining a liquid culture medium containing photosynthetic organisms;
b) removing from the photobioreactor a portion of the liquid culture medium; and
c) separating the liquid culture medium of step b) into a solid phase containing the photosynthetic organisms and a liquid phase.